The present invention relates to an optoelectronic sensing apparatus which may be used to automatically focus a camera.
Rangefinders are well known in the art of photography and the like which comprise parallel objective lenses which are spaced transversely apart from each other by the greatest practical distance. An eyepiece lens receives an image of an object from one of the objective lenses either directly or after reflection by fixed mirrors or prisms. The eyepiece lens also receives a light image of the object from the other objective lens, but after reflection by a rotatable mirror. The rangefinder is constructed so that the images from the two objective lenses coincide at one position of the rotatable mirror. Since the distance between the objective lenses and the angle of the rotatable mirror are known, the distance to the object may be calculated by simple triangulation.
In a practical rangefinder of this type, a knob is provided by which the operator may rotate the rotatable mirror, and in addition with a scale graduated in feet, meters or the like which is mechanically linked to the mirror. Thus, the operator may merely turn the knob until the two images coincide and read the distance to the object on the scale.
Where such a rangefinder is provided as an integral part of a camera, the knob is constituted by the camera focussing ring. Since the rangefinder and focussing ring are mechanically linked together and calibrated, the photographer may focus the camera by merely turning the focussing ring until the images in the rangefinder coincide, without having to know the actual distance to the object which he wishes to photograph. A variation of this type of rangefinder is the split image rangefinder in which the images from the respective objective lenses constitute adjacent vertical segments of the field of view in front of the rangefinder, with one of the segments being transversely shifted relative to the other upon rotation of the mirror.
As a natural consequence of the introduction of new electronic technology and the minaturization of electronic components, automatic systems are being developed for cameras which relieve the photographer of the troublesome tasks of setting the shutter speed, lens opening and focus. Automatic cameras are desirable for amateur photographers who do not wish to study photography in sufficient depth to learn how to manipulate the various adjustments of a camera, but still want to enjoy the advantages of good image quality and exposure. These cameras are also desirable for advanced amateur and professional photographers since they relieve them of the mundane tasks of camera operation and free them to concentrate entirely on their subject matter and composition.
Automatic exposure systems have been developed which are not the subject matter of the present invention. Rather, this invention relates to automatic focussing by means of optoelectronic sensing.
It has been proposed heretofore to provide, in the type of rangefinder described above, two identical photosensor arrays which receive the images from the first and second objective lenses. The arrays produce identical outputs at the position of the rotatable mirror at which the light images incident on the arrays are identical.
The distance to an object may be determined by rotating the mirror and sensing the position thereof at which the difference between the outputs of the arrays is minimum. The position of the mirror corresponds to the distance to the object as with the optical rangefinder. A servo system may be provided to drive a camera lens to a position corresponding to the mirror position, thereby automatically focussing the camera.
In such an optoelectronic rangefinder, two methods have heretofore been used to determine the mirror position producing the minimum difference between the outputs of the arrays. In both methods, the mirror is rotated throughout a predetermined maximum range and the outputs of the arrays compared at a number of positions of the mirror during rotation thereof, the output signals of the arrays being subtracted to produce difference signals.
In the first method the difference signals are compared with a predetermined minimum value, and the mirror position at which the difference signals first drop below the minimum value is taken as corresponding to the distance to the object. Depending on the type of scene being photographed, the difference signals following the one determined to correspond to the distance to the object may drop even further below the minimum value. The result is erroneous determination of distance and an out-of-focus picture.
The second method involves sensing the position of the mirror at which the magnitude of a difference signal is greater than the magnitude of a preceeding difference signal. Theoretically, this indicates that the minimum difference signal has been sensed and that the distance to the object has been correctly determined.
However, in photography of certain scenes such as those of trees or buildings having vertical striped patterns, the curve corresponding to the values of the difference signals has a number of inflection points. In the second method of distance determination the distance will be determined as corresponding to the first concave inflection point, even if the actual distance produces a subsequent difference signal having a lower magnitude. One skilled in the art will readily appreciate how this method can produce a determined distance which varies from the actual distance by a great extent.
Prior art optoelectronic rangefinders of the type described above may comprise analog integrators made up of operational amplifiers and other components. In order to improve the accuracy of the apparatus, the arrays each comprise a plurality of photosensor elements which are sequentially strobed a number of times during rotation of the mirror. The outputs of respective elements are applied to a differential amplifier which produces the difference signals. An analog integrator integrates the difference signals from all of the elements each time the arrays are strobed to produce a summation signal. The summation signal is compared with a predetermined minimum value or the preceeding summation signal for distance determination.
Analog integrators of this type comprise capacitors which are charged by the difference signals to an extent corresponding to the integrals of the difference signals with respect to time, in summation. After the summation process is completed at one position of the mirror, the capacitor must be completely discharged in preparation for the next summing operation.
The time constant requirements of the capacitor for integration and substantially instantaneous discharge are mutually conflicting, generally limiting the speed of operation of the circuitry. In addition, the capacitor and other elements of the circuitry are subject to wide variations in the effective values thereof caused by temperature, humidity, ageing, etc., causing inaccurate operation of the apparatus. The accuracy of the apparatus is also limited by the analog integrating components which are inherently less accurate than digital components.